Spills or leaks of hydrocarbon fuels from storage tanks, transfer lines, transports and the like may contaminate soil at the location of the spill. Removal of the hydrocarbon fuels from the soil is often required to protect the environment.
Hydrocarbon vapors which remain in empty fuel storage tanks, both underground and above ground, or transportable tanks must often be removed. Removal of the hydrocarbon vapor, or degassing, inerts the atmosphere in the tank to preclude the possibility of explosion.
The prior art techniques for accomplishing these tasks are expensive, complex, and often result in mere transfer of the environmental problem to another location. The present invention provides a method for economically decontaminating soils or degassing tanks without creating other environmental impact.
Vacuum extraction of landfill gases containing naturally produced methane has been a commonly used technology for many years. The methane collected in this manner is either flared or stored to be burned as a source of energy. Recent laboratory studies and field projects have demonstrated the feasibility of removing heavier hydrocarbon vapors such as those produced by gasoline using the vacuum extraction technique.
Vacuum applied by means of a blower or vacuum pump of common manufacture to a perforated well casing placed within the soil creates a pressure gradient in the soil surrounding the casing. Heavy hydrocarbons present in the soil are volatilized and the vapors migrate through the perforations in the casing into the casing and are drawn to the vacuum source. The hydrocarbon vapor exhausted through the pump or blower is typically discharged directly into the atmosphere.
The discharge of hydrocarbon vapors from soil contamination sites or in the process of degassing tanks may pollute the atmosphere. The development of strict air quality regulations in many parts of the United States proscribes such discharges as environmentally unacceptable.
Two prior-art technologies have commonly been used to prevent the discharge of hydrocarbon vapors. The first of these technologies uses activated carbon inside a vessel or series of vessels to adsorb the vapors. A typical adsorption ratio is 1 pound adsorbed hydrocarbon to 7 pounds activated carbon. A very large amount of activated carbon is required to adsorb a small amount of hydrocarbon vapor. Further, the contaminated carbon must itself be disposed of. Transportation of the contaminated carbon to a hazardous waste disposal site merely transfers the pollution problem from one location to another. The spent carbon may be reprocessed; however, during reprocessing, the hydrocarbon vapor must be destroyed. In either case, transportation of large quantities of spent carbon is expensive and inconvenient.
The second prior-art technology destroys hydrocarbon vapors by catalytic oxidation. The hydrocarbon vapors are introduced into a large incinerator containing a precious metal catalyst such as platinum. Catalytic conversion of large quantities of hydrocarbon vapor requires the use of large catalytic beds of complex and sophisticated design. The catalyst must accommodate a wide range of hydrocarbon concentrations commonly encountered. The catalytic oxidation process also requires the addition of external heat. Under most circumstances, this requires a supplemental heat source such as electric heating coils or a supplemental hydrocarbon fuel.
Both prior-art technologies are expensive and complex.